Until recently, most telephones contained a bell ringer operated by a 90 volt ringing signal transmitted along the telephone lines from a telephone switching office. More recently, many telephones have been made with local sound generators in which two alternating tones are reproduced in a local speaker to create a warble. Where these telephones are of digital type, for example, digitally generated signals are converted to analog, and the analog signals are reproduced in a loudspeaker. The volume of the sound is controlled in an amplifier of the analog signal.
Normally telephones provide only an alternating two tone signal which indicates to a user that there is an incoming call to the telephone. Such ringers make no provision for specialized alarm ringing at different ringing frequencies or at a different cadence than normal ringing, and cannot provide specialized ringing frequencies for calls directed to different members of the household.